This invention relates to laminated automotive glazing panels provided with solar control coating layers. Whilst the invention will be particularly described with reference to vehicle windscreens it should be understood that it may have other applications.
The use of coating layers is well know to modify the optical properties of glazings. In particular, coating layers may be used to reduce the proportion of incident solar energy which is transmitted through the glazing whilst allowing passage of sufficient visible light to ensure good visibility. This can reduce overheating of the interior of the vehicle in summer and is commonly achieved by reflection of incident solar radiation in the infra-red portion of the spectrum. Coating layers may also provide an electrically heatable element for a glazing. EP378917A (Nippon Sheet Glass Co.) discloses such coating layers.
The term solar control coating layer as used herein refers to a coating layer which increases the selectivity of the glazing panel i.e. the ratio of the proportion of incident visible radiation transmitted through the glazing to the proportion of incident solar energy transmitted through the glazing.
The term luminous transmittance as used herein means the luminous flux transmitted through a substrate as a percentage of the incident luminous flux measured using CIE Illuminant A at 2° observer.
There are a number of different families of solar control coating layers, each of which have differing properties and characteristics. These include:
a) pyrolytic coatings obtained by contacting a liquid or vapour composition with the hot surface of a glass sheet. Such coatings include tin oxide coatings doped with fluorine and indium tin oxide (ITO) coatings. Pyrolytic coatings have the general characteristic of being hard coatings (i.e. they are relatively resistant to abrasion) and of being relatively easy to handle during manufacturing processes without damage to the coating layer. Many pyrolytic coatings are inherently heat resistant to a sufficient extent to enable glass sheets to which they are applied to be bent and/or thermally tempered without significant deterioration of their solar control properties. A significant industrial advantage results from the ability to apply pyrolytic coatings to a continuous ribbon of flat glass for example as part of a process of making float glass. European patent application EP 353 141 A (Saint Gobain Vitrage) describes a heatable pyrolytic indium tin oxide coating on face 3 of a laminated windscreen. Such glazings have never found commercial success in the automotive field due particularly to the inherent optical and energetic limitations of this type of coating.
b) Sputtered single silver layer coatings obtained by sputtering a silver containing layer on to a supporting substrate. Such coatings usually comprise a coating stack having the general form: supporting substrate antireflective layer optional barrier layer/silver containing layer/optional barrier antireflective layer. In such a structure the silver containing layer serves to reflect radiation in the infra red portion of the spectrum, the antireflective layers serve to reduce reflection of light in the visible portion of the spectrum that would otherwise be caused by the silver containing layer and the optional barrier layers serve to protect to silver continuing layer either during deposition of the coating and/or subsequent processing. Whilst the optical performance of single silver layer sputtered coatings is reasonably good such sputtered coatings are general “soft” coatings i.e. they are not particularly resistant to abrasions and scratches and require significant care in handling to avoid damage. In addition, significant care in both the design and handling of such layers is required to enable them to be sufficiently heat resistant to allow tempering and/or bending of a substrate to which they are applied.
c) Sputtered double silver layers obtained by sputtering two, spaced silver layers onto a supporting substrate. Such coatings usually comprise a coating stack having the general form: supporting substrate/antireflective layer/optional barrier layer/silver containing layers optional barrier layer/antireflective layer/optional barrier layer/silver containing layer/optional barrier antireflective layer. In such a structure the sliver containing layers serve to reflect radiation in the infra red portion of the spectrum, the antireflective layers serve to reduce reflection of light in the visible portion of the spectrum that would otherwise be caused by the silver containing layer and the optional barrier layers serve to protect to silver containing layers either during deposition of the coating and/or subsequent processing. The infra red reflective silver containing layers are commonly layers of silver or a silver alloy have a thickness in the order of 80 to 120 Å. The optical performance of double silver Layer sputtered coatings can be extremely good, especially in terms of their selectivity but perhaps even more so than with single silver sputtered coatings these coatings are extremely fragile both in terms of resistance to abrasions and scratches (for example during handling) and in their ability to withstand heating for example to enable them to be sufficiently heat resent to allow tempering and/or bending of a substrate to which they are applied.
One example of the use of sputtered coating layers In automotive applications is U.S. Pat. No. 4,668,270 (Ford Motor Company) which describes a car windscreen having an electrical heatable coating layer used for defrosting, de-icing and/or de-misting. The heatable waling, which is laminated between the two glass sheets of the windscreen, is supplied with electrical power via first and second bus bars which extend respectively along the top and bottom edges of the windscreen, each bus bar being silk screen printed on the glass in a silver ceramic material. The heatable coating is a multilayer coating consisting of zinc oxide and silver formed by magnetron sputtering.
The physical nature of double silver layer coatings layers is entirely different to tat of, for example, pyrolytic coating layers and, consequently, entirely to different techniques must be employed for their design, processing and use.
It has generally been believed in the art that the precautions of the techniques described below must be adhered to to enable the successful use of sputtered double silver layers In laminated automotive glazings:
1) that the sputtered double silver layers should be deposited on a carrier film of, for example pet (poly ethylene tetrachloride), which is assembled between the to glass sheets of a laminated glazing once the individual sheets have been bent to their desired final shape. One disadvantage of such carrier films is the difficulty of ensuring that the film correctly follows then precise contour of the bent glazing panel. Consequently, this procedure is limited to use with glazing panels of a relatively simple curvature. In addition, it is generally not desirable to electrical heat the solar control coating in such an arrangement due to deterioration of the coating and/or of the carrier film and it is also inconvenient to provide bus bars in this arrangement to relay electrical power to the coating. Consequently, this technique is generally unsuitable for use with heatable windscreens.
2) that, alternatively, the sputtered double silver layers should be applied to the concave face of a pre-bent sheet of glass prior to its assembly to form a laminated glazing panel. In this way, the coating layer is not subjected to the heat treatment necessary to form the desired curvature of the glass sheet disadvantages of his technique include the technical difficulty of sputter depositing coating layers onto a curved sheet of glass so as to ensure that the entire glass surface is evenly coated (due, amongst other things, to the variation in the distance between the different part of the glazing surface and he targets used for the sputtering process-small variations in thickness of the coating layers can cause undesirable colour variations across the glazing panel) and the complexity and limitations (including dimensional limitations-complex windscreens having deep curvatures will not always fit in to such coating machines simply because of their dimensions) of coaters which can sputter deposit layers onto a curved substrate. Consequently, this technique is also limited to use with relatively simple curvatures of glazing panels.
3) alternatively, sputter depositing a double silver coating layer onto a relatively flat sheet of glass and subsequently bending the glass sheet carrying the coating stack to its desired shape prior to assembly as a laminated glazing panel Due to the fragility of this type of coating, the glass sheet carrying the coating stack should be bent such that the coating stack is at the concave face of the curved sheet of glass. This is so that the layers of the coating stack have a tendency to be compressed during the bending process so as to ensure the integrity and continuity of the to layers of the coating stack, this is particularly so for complex curvatures of glaring